Manufacturing nitrocellulose



Patented July 9, 1946 MANUFACTU ING NITROCELLULOSE Frederick J. Bouchard, Radford, Va., Robert C. Gunness, Chicago, Ill., and Raphael L. Stern, South River, N. J., assignors to Hercules Powder Company, Wilmington, Del., a corporation v of Delaware Application May 19, 1943, Serial No. 487,558

12 Claims. 1

This invention relates to the manufacture of nitrocellulose and more particularly it relates to the recovery of spent nitrating acid adhering to nitrocellulose after separation of the nitrocellulose from the major portion of the spent acid formed in the nitration step.

Nitrocellulose is prepared by nitrating cellulose in a large excess such as 30-65 times by weight of the cellulose of mixed acid. Mixed acid is a mixture of nitricfacid, sulphuric acid and a relatively small proportionof water. The spent acid from the nitration is slightly weak-v ened in nitric acid and slightly increased in water content. It is absolutely essential that it be recovered for fortification and re-use. The major part of the spent `acid is relatively easily separated from `the nitrocellulose. However, it is quite di'cult to recover the-remaining portion wetting the nitrocellulose without damaging the quality of the nitrocellulose.

It has been proposed to wash nitrocellulose containing strong spent acid with progressively more dilute spent acid so as to lose a minimum of nitric acid. This method has the serious objection that the nitrocellulose is subjected to denitrating conditions and becomes nonuniform and variable in ultimate degree of nitration. It has also been proposed to use wash water repeatedly until its acid concentration permitted treatment with ferrous sulphate or like reducing agent to recover nitric oxide. This method has the objec- 'tion of nonuniform treatment of succeeding batches of nitrocellulose, is inefficient in nitric acid recovery, and loses sulphuric acid present. These and other methods for recovery of lspent acid have either subjected the nitrocellulose to varying periods of treatment with acid Vor to varying concentrations causing serious nonuniformity.

Thus, in practical operation, nitrated cellulose has been removed from supernatant acid by centrifuging, squeezing or the like, to obtain nitrocellulose wet with one to three times its weight of adhering spent acid. Thismaterial has then been drowned in a large volume of water to decrease the concentration of the'acid in contact with the nitrocellulose to below-2 or 3% very rapidly to prevent denitration by intermediate strength acid. The retained or adhering spent acid has been lost and has caused disposal difcu-lties since it has been diluted too greatly for recovery. I

' The process according to this invention is concerned with the recovery ofthe spent acid adhering-to nitrocellulose after its separation from' Y mixed acid (nitric and sulphuric acids), having a concentration between about 10% and about 25%, the volume thereof being sufficient to reduce the concentrationof the resulting acid in and on the nitrocellulose from above about to between about 10% and about 25% substantially instantaneously. The nitrocellulose is then removed from the drowning acid and spent acid` remainingtherein may be displaced, or otherwise removed by water or other washing liquid essentially water, or both. The dilute acid formed by such washing may be added to the drowning acid to permit and to aid in maintaining the concentrationY in the 10 to about 25% range. Nitric acid is evaporated from a portion ofy the drowning acid to recover a nitric acid fraction. A sulphuric acid fraction may be recovered from the evaporation residuum.

It has been found that the drowning acid of- 10-25% concentration unexpectedly has substantially no denitrating effect, i. e., denitration is negligible in the period of a few minutes or few hours between drowning and complete separation of the drowning acid. At the same time it has been found practical to recover the nitric acid and sulphuric acid from the drowning acid by evaporative fractionation. The process in accordance with this invention may be more completely described in connection with the accompanying drawing which illustrates the process diagrammatically.

Referring to the drawing the flow of pertinent material is shown as originating in a nitrator I from which the nitration mixture passes to'a separator 2, which may be a centrifuge, squeezeroll equipment or the like, and which removes the greater part of the spent acid from the nitrated cellulose. The free spent acid so obtained is led to storage for fortification and reuse. The nitrocellulose with adhering spent acid passes to the drowning equipment 3 Where it is rapidly mixed with a large volume of drowning liquor, which reduces the concentration of the acid wetting the nitrocellulose substantially instantaneously to below denitrating'levels. The nitrocellulose and drowning acids pass to a rotary filter 4 which separatesthenitrocellulose from the drowning acid. Portions of drowning acid remaining in the nitrocellulose after-the initial separation are washed with water, or with vapor condensate, which is essentiallyfwater, or preferably, first with the vapor condensate and then further with water, the vapor condensate being obtained as hereinafter described. The vapor condensate and/or water form a dilute acid in the washing operation, which dilute acid is admixed with separated drowning acid. The drowning acid flows from the filter back to the drowning equipment either directly7 or, preferably, through sedimentation equipment or a filter to remove nitrocellulose iines. The washed nitrocellulose passes from the filter into a slurry tank in which Vit is suspended in water for subsequent purification usually in a series of tubs represented as purification equipment 6.

A portion of the drowning acid separated from the nitrocellulose, preferably a quantity about equivalent to that added in the nitrocellulose and Wash liquid, is bled off and passed to a concentrator which consists of a concentration column 8 and an operatively associated evaporator 9 equipped with a suitable heating coil. The concentralor separates the drowning acid portion into the vapor fraction, essentially Water vapor with a small content of nitric acid, and a liquid fraction of mixed acid of increased concentration. This mixed acid passes` to an evaporator l0 equipped with a suitable heating coil. The major portion of the nitric acid in the mixedacid is evaporated from the drowning acid portion in this evaporator to form a vapor fraction containing nitric acid and water. The liquid residuum leaving the evaporator is apartiallyV concentrated sulphuric acid fraction containing a small amount of nitric acid. This liquid passesto a sulphuric acid denitrator i3 where, bythe action of steam, remaining vnitric acid is evaporated to form' an aqueousnitric acid vapor. The sulphuric acid liquid 4fraction leaves in a denitrated form. The aqueous nitric acid vapor leaving the evaporator I0 and the nitric acid vapor leaving the denitrator i3 pass with partial or no condensation to a nitric acid concentrator which consists of the concentrator column H and an operatively associated evaporator l2 provided with a suitable heating coil, The nitric acid concentrator resolves the nitric acid fractions fed thereto into a liquid concentrated nitric acid fraction and a vapor Vfraction consisting essentially of water 4 vapor but in most cases containing a `small amount of nitric acid. The vapors from one or both of the concentrators 8 and ll may be led to a condenser I4 where theyform a vapor condensate containing a small amountof nitric acid. This condensate, preferably cooled, may then be used in Washing nitrocellulose on the rotary filter 4, thereby permitting complete recovery of the drowning acid from the nitrocellulose and recovery of the small amount, of nitric acid in the vapor condensate. The columns 8 and Il may be of packed or bubble-cap construction.

In a typical operation serving as an example of the process, cellulose is nitrated with 45 times its weight of mixed acid to form a spent mixed acid containing about 25% nitric acid, about 55% sulphuric acid and about 20% water. The nitration mixture is centrifuged, eliminating the major portion of the spent acid, but leaving a quantity in the nitrocellulose of about 1.1 times the weight ofthe nitrocellulose. The wetnitrocellulose is then drowned with mixed 4acid dilutedl to 20% total acid concentration in a quantity of approximately 50 times the weight ofthe nitrocellulose (dry basis). The increase in concentration in the drowning acid produced by the strong. acid wetting. nitrocellulose is about 2%.

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The nitrocellulose is removed from the drowning acid by a rotary filter, washed on the iilter by displacement first with a water vapor condensate containing about 1% nitric acid and then by a small amount of pure Water. It is then treated, in thefusual manner for stabilization, viscosity adjustment and dehydration. The drowning acid is recycled through a sedimentation tank removing most of any associated nitrocellulose fibers. A portion of the drowning acid, substantially equivalent to the quantity added by the strong acid wetting the nitrocellulose and the Wash water, is withdrawn and initially concentrated tovabout 45% total acid concentration. It is .then evaporated to strip it of nearly all of its nitric acid to form a nitric acid water vapor fraction equivalent to about 30% nitric acid and a liquid residual fraction of about 65% sulphuric acid. The sulphuric acid is denitrated to free it of residual nitric acid by steam to yield a oki-68% sulphuric acid. The nitric acid vapors are concentrated to form a concentrated nitric acid of (S5-67% `nitric acid content and a vapor essentially water but containing about 1% of nitric acid. Thevapors from the concentrators are condensed, cooled and used as nitrocellulose wash Water to recover the nitric acid content of the drowned filtered nitrocellulose and to aid in maintaining the concentration of the drowning acid in the range desired.

It will be appreciated that it is not essential to use vapor condensate for Washing thev nitrocellulose. However, such operation permits substantial recovery of the small nitric acid content of the vapor condensate and it further provides a means of maintaining the drowning acid in the' lll-25% rangein which it is effective in drowning the nitrocellulose and at the same time recoverable. The vapor condensate in general will include nitric acid in any quantity up to about 3%. It will be appreciated that the vapor condensate may be returned directly to the drowning acid without use for washing.

Under most conditions of operation, it is necessary to add small quantities of water to the system to prevent the drowning acid from building up in concentration to a level at which denitration of nitrocellulose aft-er drowning ceases to be negligible. Such make-up water is conveniently added by using pure water as a final wash for thenitrocellulose on the filter and merging the resultant Wash water with the drowning acid.

By this expedient, substantially all of the drowning acid is removed and recovered from the nitrocellulose.

The nitric acid may be recovered by distillation from the 10-25% mixed drowning acid and subsequently further concentrated. However, it has been found that preconcentration to form a mixed acid of about 35% to about 60% concentration greatly Aimproves the eiciency of the steps of evaporating or distilling the nitric acid and subsequently concentrating it. Such preconcentration Yalso makes practicable subsequent denitration of the residual sulphuric acid. The concentration is accomplished by evaporation of essentially water from the acid.

Usually the aqueous nitric acid fraction obtained is concentrated to (iO-68%. The fraction may first be condensed but Ypreferably the vaporous fraction with partial or no condensation is introduced directly into a concentrating column. Thev sulphuric acid fraction is conveniently brought to 50-'75%` as denitrated and may then be further concentrated in other equipment when stronger'. acid is desired. The denitration' step separates the small residual quantity of nitric acid, which is less than of the residual fraction, from the sulphuric acid to permit. ready concentration of the sulphuric acid.

It is essential that the concentration of th drowning acid be held within therange hereinabove specified. The concentration must be no greater than about 25% since otherwise denitration of the nitrocellulose 'occurs during and after drowning in a variable and unpredictable manner. `At below concentrations of 25% and with use of vthe drowning acid in a volume to insure that the mixture resulting is below about 25%, substantially'no denitration occursand furthermore, the nitrocelluloseobtained is highly uniformxfrom'batch to batch. Concentrations of drowning acid below about donot permit obtaining the economies of the present invention since 'recovery ofthe component nitric and sulphuric acids fromthe acid is impractical. A concentration between and 20% is preferred.

, The mixed acid wetting the nitrocellulose after its removal from the major portion of the spent acid will have a total acid concentration in excessrof about 75%. The ratio of sulphuric acid to nitric acid will be between about 4: 1 and about 0.5:1, usually between 3:1 and 1:1', by weight. 'Ihe same relationship between sulphuric acid and nitric acid will apply in the drowning acid.

The electiveness of `the present process in economic recovery of spent acid is in no small part dependent upon recovery of the drowning acid wetting the drowned nitrocellulose by means ofthe rotary lilter which provides a filter surface through which the drowning acid may be drained, and which further provides for eicient displacement of residual acidthroughthe lter surface with the essentially pure water of the condensedvapor obtained from the 'acid concentrators and with the iinal wash of pure water. The Viilter further provides a very convenient manner for introducing an amount of water to the drowning acid suiicient or nearly suicient to maintain the concentration of the drowning acid in the critical range by compensating for the effect of the addition of strong acid in the nitrocellulose. However, it will be appreciated that water added as such or in the formoi dilute acid will be in a quantity no more than that quantity which is required to maintain the concentration at the desired level in the critical range.

By maintenance of the drowning acid substantially constant in concentration, a highly uniform nitrocellulose product is obtained. It has been found that the method of this invention produces a uniform nitrocellulose of a quality fully equivalent to material obtained by drowning as heretofore with water involving complete loss of retained acid.

The present process is'of particular usefulness in permitting direct and simple recovery ofthe nitric acid contained in the retained spent acid, and furthermore it leads to recovery of both the nitric acid and the sulphuric acid therein. AIn addition, the problem of Vdisposal of waste dilute acid is avoided. Y

All acid concentrations herein and in the claims are in percent by weight. u

What we claim and desire to protect by Letters Patent is: .v

1. In a procedure forl manufacturing nitrocellulose in which cellulose is nitrated in Yan excess'of mixed acid `.and the resulting nitrocellu- .v6 loseiis separated from supernatant spent mixed acidof-above about '75% total acid concentration by weight, drowned to reduce the concentration of acid wetting thenitrocellulose rapidly to below. concentrations causing substantial denitration, and then .puried a process which comprises'carrying out the drowning of the said separated nitrocellulose wet with spent acid by mixing it with a quenching volume of aqueous mixed nitric and sulphuric acid having a concentration of about 10 to about 25% by'weight of total acid, said'volume'being sufficient to reduce .the concentration of the spent acid on and .in the nitrocellulose to between about 10% and about"25% substantially instantaneously, removing the drowned nitrocellulose from the drowning acid, and evaporating nitric acid from a portion of the drowning acid.

2.' In a procedure for manufacturing nitrocellulose in which cellulose is nitrated in an excess of mixed acid and the resulting nitrocellulose is separated from supernatant spent mixed acid of above about total acid concentration by weight, drowned to reduce the concentration ofacid wetting the nitrocellulose rapidly to below concentrations causing substantial denitration, and then purified, a process which comprises carrying out the drowning of the said separated nitrocellulose wet with' spent acid by mixing it with a quenching volume of aqueous mixed nitric and sulphuric acid having'a concentration of about 10 to about 25% by weight of total acid', said volume being sucient to reduce the concentration of the spent acid on and 'in the nitrocellulose to between about 10% and about 25% substantially instantaneously, removing the drowned nitrocellulose from the drowning acid, washing the nitrocellulose with essentially water thereby obtaining a dilute acid, adding said dilute acid to the drowning mixed acid in a quantity no more than that maintaining the drowning acid between about 10% and about 25% in total acid concentration, and evaporating nitric acid from a portion of the drowning acid.

3. In a procedure for manufacturing nitrocellulose in which cellulose is nitrated in an excess of mixed acid and the resulting nitrocellulose is separated from supernatant spent mixed acid of above about 75% total acid concentration by weight, drowned to reduce the concentration of acid wetting the nitrocellulose rapidly to below concentrations causing substantial denitration,

and then puried, a process which comprises car-` rymg out the drowning of the said separated nitrocellulose wet with spent acid by mixing it with a quenching volume of aqueous mixed nitric and sulphuric acid having a concentration of about 10 to about 25% by weight of'total acid, said volume being sufficient to reduce the concentration of the spent acid on and in the nitrocellulose to between about 10% and about 25% substantially instantaneously, removing the drowned nitrocellulose from the drowning acid, washing the nitrocellulose. with a condensed vapor essentially water thereby obtaining a dilute acid, addacid of above about '75% total 'acidconcentration by weight, drowned to reduce the concentration vof .acid wetting the'nitroce'llulose rapidly to below concentrations causing.substantialv de-V nitra'tion, and then purified, alprocessiwhich comprises icarrying .out the drowning :of the .saidrsepe aratedxnitrocellul'ose `wet with spent .acid by mixing it with a quenching volume .of .aqueous mixed nitric and sulphuric acid having aconcentration of about 10 to about 25% by Weight of total acid, said volume being suiiicient to reduce the concentration of the spent acid'on and in the nitrocellulose to betweenabout 10% and about25% substantially instantaneously, :removing the drowned nitrocellulose from the drowning acid, evaporating a portion of the drowning acidltoj form-a fraction o f essentially water vapor, 'condensing the Vapor fraction, and returning condensed Vapor to the drowningv acid in an `amount permitting the concentration of the drowningv'acid to be `maintained between about 10% and about 25% Vby Weight.

v5. In a proceduce .for manufacturingnitrocellulose in which cellulose is nitrated in an excess of.mixed acid and the resulting nitrocellulose is .separated from supernatant spentmixed acid of above about 75% total acid concentration by weight, drowned to reduce the concentration of acid wetting the nitrocellulose rapidly to below concentrations causing substantial denitration, and then pur'iiied, a process which comprises .carrying out the drowning ofthe said separated nitrocellulose wet with spent acid by mixing it with a quenching `volume of aqueous mixed nitric 'and sulphuric acid having a concentration of about 10 to about 25% by weight of total acid, said volume being suiiicient to reduce the concentration ofthe spent acid on and in the nitrocellulose to between about 10% and about 25% substantially instantaneously', removing the drowned nitrocellulose from thewdrowning acid, evaporating a portion of the drowning acid -to form a .nitric acid fraction, concentrating .the .nitric acid fraction to form a concentrated acid-fraction and a .fraction essentially Water vapor, condensing the said water vapor fraction, Vand returningtheresulting. .condensate .to thedrowning acidrin an amount permitting the concentration of the drowning acid to be maintained between about 10%. and about .25% by Weight.

6. In a procedure .for manufacturing nitrocellulose in which cellulose is .nitrated' in van excess of mixed acid and the .resulting-.nitrocellulose is separated from supernatant spent mixed acid :of

above about '75% total acid concentration by 5 weight, drowned to reduce the. concentration Yof acid wetting the nitrocelluloserapidly to vbelow concentrations causing substantial denitration, and thenpuriiied, a process which .comprisescar rying out the drowning of 'the said separated .nitrocellulose wet with spent acid bymixing it with a quenching volume of aqueous mixed :nitricandsulphuric acid having a concentration of about 10 to about 25% by weight of total. acid, saidfvolurne being sufficient to reduce. the concentration of the spent acid on andl inthe nitrocelluloseto between about 10%. and` about 25%.substantially instantaneously, removing-the drowningacid from: the nitrocellulose fibers -bydrainagethrough aA filter surface and by displacement .of;residualacid from the nitrocellulose through fthe :iilter :surface with essentially water, and .evaporating nitric acid from .a portion of-.the drowning acid.;

7. y In Va procedure for 'manufacturing :nitrocel-.y lulose .in which celluloseV is nitrated 'in :an excessv 8 ofmixed .acid and .fthe resulting .nitrocellulose is separated fromisupernatant spent mixedacidiof above about 75% total .acid concentration by Weight, drowned to .reduce .thefconcentration of acid wetting the nitrocellulose rapidlyto. below concentrations causing substantial denitration, and. then puried,` aprocess .which comprises carrying 'out fthe. drowning yof the .said separated vnitrocellulosev wet with .spent acid .by mixing 'it .with aquenching 'volume of .aqueous .mixed .nitric and sulphuric acid having a. concentration vof :about 10.-to about-25% byzweight of .total acid, said volume .being :suicien't .toreduce .the concentration of thespent .acid `on and. in the .nitrocellulose to betweenabout.l0%.and about .25% substantially instantaneously, .removing :the drowned nitrocellulose Vfrom thedrowningacid, .evaporate ing nitric acid from a .portion of the drowning acid, :and recovering a ysulphuric acidiractionof increased concentration frornthesaid portion;

8; In aprocedurefor manufacturingnitrocellulose .in which cellulose is nitrated .inv an excess 4:of mixed acid and the .resultingnitrocellulose is separated from supernatant spent mixed acid of above about 75% total acid concentration by weight, drowned to'reduce the concentration `.of acid wetting the nitrocellulose rapidly `to .below concentrations causing substantial denitration, and then purified, a process which comprises carrying out the drowningof thesaidseparated .nitrocellulose wet with spent acidby mixing it .with a quenching volume of aqueous lmixed Anitric and sulphuric acid havingfa concentrationof .about l0 to about 25% by weight of totalacid, said volume being sufficient to reduce the/concentration of the spent acid on and Vin the` nitrocel1ulose to between about 10%` and about 25% substantially instantaneously, removing ,thedrowned nitrocellulose. from the drowning acid, concentrating a portion of the drowning acid, andevaporating .nitric acid from the concentratedportionto recover a nitric acid fraction and a sulphuric .acid Aresidual fraction.

9. In a `procedure for manufacturing .nitrocellulose .in which cellulose is nitrated in .an excess :of mixed acid and the: resulting nitrocellulose .is separated from supernatant spent .mixed acid of abovev about 75% totalacid concentration .by Weight, drowned to reduce the concentration .of acid wetting'the nitrocellulose. rapidly to below concentrations causing; substantial 'denitration., and then purified, a process which comprises carrying out the drowning of the said separated nitrocellulose wet withV spent acid .by mixing it with a quenching volume oaqueous .mixed nitric and sulphuric acidv having a concentration of about 10 to about V25% .by weight'of totalaci'd, vsaid volume .being `suiicient to reduce the concentra'- tionof thefspent acid. yon andin'the nitrocellulose to between about 10% and. about 25% substantially instantaneously, removing the .drowned .ni-

trocellulose from. the drowningacid, concentrating-a portion ofthe drowning acid, and evaporating nitric acid .from the concentrated portion to recover-.a nitricv acid fraction and a sulphuric acid and then puried, a process which comprises carrying out the drowning of the said separated nitrocellulose wet with spent acid by mixing it with a quenching volume of aqueous mixed nitric and sulphuric acid having a concentration of about to about 25% by weight of total acid, sai-cl volume being sufficient to reduce the concentration of the spent acid on and in the nitrocellulose to between about 10% and about 25% substantially instantaneously, removing the drowned nitrocellulose from the drowning acid, concentrating a portion of the drowning acid, subjecting the resulting acid to evaporation to form a vapor fraction of water and a major portion of the nitric acid in the drowning acid concentrate and a liquid fraction of aqueous sulphuric acid containing the remaining nitric acid, denitrating the sulphuric acid fraction to form an aqueous nitric acid vapor, and subjecting the tWo said nitric vapors to concentration to form and recover a concentrated nitric acid fraction.

11. In a procedure for manufacturing nitrocellulose in which cellulose is nitrated in an excess of mixed acid and the resulting nitrocellulose is separated from supernatant spent mixed acid of above about '75% total acid con-centration by weight, drowned to reduce the concentration of acid wetting the nitrocellulose rapidly to below concentrations causing substantial denitration, and then purified, a process which comprises carrying out the drowning of the said separated nitrocellulose wet with spent acid by mixing it with a quenching volume of aqueous mixed nitric and sulphuric acid having a concentration of about 10 to about 25% by weight of total acid, said volume being suicient to reduce the concentration of the spent acid on and in the nitrocellulose to lbetween about 10% and about substantially instantaneously, removing the drowned nitrocellulose from the drowningacid, concentrating a portion of the drowning acid to form an acid concentrate fraction and a vapor fraction essentially water vapor with a small content of nitric acid, subjecting the acid concentrate to evaporation to form an aqueous nitric acid vapor fraction and a residual fraction containing the 'sulphuric acid of the drowning acid portion 10 treated, concentrating the nitric acid fraction to form a concentrated nitric acid fraction and a second vapor fraction essentially water vapor with a small content of nitric acid, condensing the two essentially Water vapor fractions, and returning the condensate to the drowning mixed acid in an amount permitting the total acid concentration of the drowning acid to be maintained between about 10% and about 25%.

l2. In a procedure for manufacturing nitrocellulose in which cellulose is nitrated in an excess of mixed acid and the resulting nitrocellulose is separated from supernatant spent mixed acid of above about total acid concentration by weight, drowned to reduce the concentration of acid wetting the nitrocellulose rapidly to below concentrations causing substantial .denitration, and then puried, a process which comprises carrying out the drowning of the said separated rlitrocellulose wet with spent acid by mixing it with a quenching volume of aqueous mixed nitric and sulphuric acid having a concentration of about 10 to about 25% by weight of total acid, said volume being suilicient to reduce the concentration of the spent acid on and in the nitrocellulose to between about 10% and about 25% substantially instantaneously, removing the drowned nitrocellulose from the drowning acid, concentrating a portion of the drowning acid to form an acid concentrate fraction and a vapor fraction essentially water vapor with a small content of nitric acid, subjecting the acid concentrate to evaporation to form an aqueous nitric acid vapor fraction and a residual fraction containing the sulphuric acid of the drowning acid portion treated, concentrating the nitric acid fraction to form a concentrated nitric acid fraction and a second vapor fraction essentially water vapor with a small content of nitric acid, condensing the two essentially water vapor fractions, washing nitrocellulose Wet with residual drowning acid to remove said residual acid and to form therewith a diluted acid, and adding said diluted acid to the drowning acid.

FREDERICK J. BOUCHARD. ROBERT C. GUNNESS. RAPHAEL L. STERN. 

